1. Field of the Invention
The present invention relates to an optical transmission apparatus that transmits optical signals by wavelength-multiplexing the optical signals in an optical network.
2. Description of the Related Art
In a conventional backbone optical network (a metro core) that connects between cities, a wavelength division multiplexing (WDM) technique is used. In the WDM technique, optical signals are transmitted by multiplexing the wavelengths corresponding to the number of channels necessary for transmission paths, using an optical add and drop multiplexer (OADM) that is installed in each city.
FIG. 8 is a schematic of an OADM in a multi-stage connection in a backbone optical network. In a backbone optical network 9, plural N units (10a to 10n) of OADMs 10 are disposed on an optical transmission path (an optical fiber 140) to transmit optical signals. The backbone optical network 9 is, for example, a ring network. Each OADM 10 (10a to 10n) includes an optical amplifier 11 that amplifies a wavelength division multiplexed (WDM) light output from the optical fiber 140, a add/drop multiplexing unit 20 that adds and drops an optical signal for each channel of the WDM light, and an optical amplifier 12 that amplifies the added or dropped WDM light, and inputs the amplified WDM light to the optical fiber 140.
In the backbone optical network 9 shown in FIG. 8, the OADM 10a at a position A adds optical signals of five waves, the OADM 10b at a position B adds an optical signal of one wave, and the OADM 10n at a position N drops optical signals of five waves. Each OADM 10 on the backbone optical network 9 frequently adds and drops optical signals. In some cases, the optical fiber 140 at a position X between the position A and the position B is broken, and only one optical signal of one wave is transmitted to the optical fibers 140 at the position B and subsequent positions. Even when the number of operating wavelengths changes frequently, the backbone optical network 9 (the OADM 10 (10a to 10n)) needs to maintain communication quality for each optical signal.
To maintain communication quality, the optical amplifiers 11 and 12 in the OADM 10 control the gain of the input WDM light at a constant level. Specifically, regardless of the number of operating wavelengths of the WDM light, the output gain of each signal is set constant.
FIG. 9 is a schematic of the add/drop multiplexing unit. Assume that the WDM light is transmitted to a direction of an arrow A shown in FIG. 9. An add/drop multiplexing unit 20 includes a drop port 21, a demultiplexing filter 22, a variable optical attenuator (VOA) 23, a multiplexer 24, and an add port 25. FIG. 10 depicts an output of the WDM light. The horizontal axis represents a wavelength (λ), and the vertical axis represents power (Po) of each optical signal of the WDM light. As shown in FIG. 10, the WDM light is transmitted while optical signals of wavelengths λa to λn and an amplified spontaneous emission light (ASE) are accumulated as noise in the whole area.
When the WDM light as shown in FIG. 10 is input to the OADM 10, the optical amplifier 11 amplifies the WDM light, and outputs the amplified WDM light to the add/drop multiplexing unit 20. When the add/drop multiplexing unit 20 passes the whole optical signals without adding or dropping an optical signal, the demultiplexing filter 22 demultiplexes the signals into signals of wavelengths, and inputs the optical signals to the variable optical attenuator 23. In this case, light other than the optical signals of the wavelengths λa to λn is not demultiplexed. Therefore, the ASE other than those near the optical signals of the wavelengths λa to λn is removed. The variable optical attenuator 23 attenuates the optical signals of the input optical signals of the wavelengths λa to λn, thereby correcting the signal waveform and adjusting the output level, and outputs the corrected result to the multiplexer 24. The multiplexer 24 multiplexes again the optical signals demultiplexed into those of different wavelengths, and outputs the multiplexed optical signals to the optical fiber 140.
FIG. 11 depicts the WDM light output from the add/drop multiplexing unit. The horizontal axis represents a wavelength (λ), and the vertical axis represents power (Po) of each optical signal of the WDM light. As shown in FIG. 11, the WDM light output from the add/drop multiplexing unit 20 includes the optical signals of the wavelengths λa to λn, and the ASEs (λa_ase to λn_ase). In this way, when viewed from a certain OADM 10, the output of each optical signal is set constant to maintain the communication quality (see, for example, Japanese Patent Application Laid-Open No. 2001-111495).
However, according to a conventional OADM, when the number of operating wavelengths changes while the OADMs are connected at multiple stages on the backbone optical network 9, a transitional delay occurs in the gain control immediately after this change, and the communication quality degrades during this transitional period.
FIG. 12 is a schematic for illustrating a change in the number of operating wavelengths of the WDM light input to the optical amplifier. The horizontal axis represents a wavelength (λ), and the vertical axis represents power (Po) of each optical signal of the WDM light. FIG. 12 depicts the change of states from a time 1201 to a time 1202 after a lapse of a time t. It is explained below that optical signals of six operating wavelengths λa to λf of the WDM signals input to the optical amplifier 11 or the optical amplifier 12 at the time 1201 change to an optical signal of only one wave λf at the time 1202.
FIG. 13 depicts a change in the wavelength when optical signals having six waves change to an optical signal having one wave in the conventional OADM. The horizontal axis represents a wavelength (λ), and the vertical axis represents power (Po) of each optical signal of the WDM light. FIG. 13 depicts the change of states from a time 1301 to a time 1302 after a lapse of a time t.
Assume that the WDM light including the optical signals having six operating wavelengths λa to λf at the time 1201 as shown in FIG. 12 is input to the OADM 10. When the optical signals pass through the demultiplexing filter 22, the ASE other than the optical signals is removed as described above. Therefore, the WDM light including the optical signals having the six wavelengths λa to λf, and the ASEs (λa_ase to λf_ase) is output at the time 1301.
On other hand, assume that the number of the operating wavelengths of the WDM light is changed to one, and the WDM light including the optical signal of only one wave as shown at the time 1202 in FIG. 12 is input to the OADM 10. When the optical signal passes through the demultiplexing filter 22, the WDM light at the time 1302 as shown in FIG. 13 is output. At the time 1302, the WDM light includes the optical signal having the wavelength λf, and the ASE (λa_ase to λf_ase). This WDM light with these wavelengths is input to the optical amplifier 12. In this case, the optical amplifier 12 detects the power of the whole wavelengths of the optical signals to control the gain. In other words, the optical amplifier 12 controls the gain so that the gain becomes constant based on the power of the ASE (λa_ase to λf_ase) in the state of the optical signals of the six waves immediately before the number of the operating wavelengths transitionally changes, immediately after the number of the wavelengths of the optical signals becomes one.
FIG. 14 is a schematic for illustrating the output of the optical amplifier in the change of the operating wavelengths shown in FIG. 12. The horizontal axis represents a wavelength (λ), and the vertical axis represents power (Po) of each optical signal of the WDM light. FIG. 14 depicts the change of states from a time 1401 to a time 1402 after a lapse of a time t. Assume that in the state shown in FIG. 13, the optical signal of one wave λf has predetermined power among the six waves λa to λf having the power as shown at the time 1401 in FIG. 14.
Assume that thereafter, the number of operating wavelengths changes, and the five waves λa to λe are excluded, and only one wave λf is available. During a transitional response period, immediately after the number of the operating wavelengths changes, the optical amplifier 11 (12) decreases the intrinsic power of the one wave λf by a portion indicated by an arrow D to the power shown at the time 1402. The optical amplifier 11 (12) controls the gain in the form of averaging the whole wavelengths λa to λf at the time 1041. Therefore, the optical amplifier 11 (12) controls the gain such that the power of the one wave λf at the time 1402 is affected by the power level of the five waves λa to λe at the time 1401 when the power levels are substantially arranged.
From the viewpoint of one OADM 10, such power, in other words, the gain variation, can be disregarded as a minor gain variation. However, if the OADMs 10 are connected in multiple stages as the OADMs 10a to 10n in the actual backbone optical network 9 as shown in FIG. 8, as optical signals are dropped from and added to the OADMs 10 toward the latter stages on the transmission path, this gain variation is gradually accumulated. When the accumulated gain variation becomes too large, the gain of optical signals becomes smaller than that of the reception ranges, and the optical signals cannot be recognized. As a result, a reception error occurs, and communication quality is degraded.
The OADM disclosed in Japanese Patent Application Laid-Open No. 2001-111495 has a function of preventing reduction in the gain level. However, the function requires an additional light source to be provided in the OADM. This increases both cost and size.